The process described above is known in the technical literature as the module jet concept, cf. Das Papier, Number 10A (1995), pages V 99-V 105, and Wochenblatt fur Papierfabrikation, 122 (1994), 485-491. With the aid of this special head box, it is possible to produce, inter alia, high-quality supercalandered papers (SC papers) having a high filler content and a uniform formation. However, the papers obtainable by this process have a tendency to dusting.
The paper machines usually have only a single stream for feeding the paper stock to the head box. In addition to the fibers suspended in water, the paper stock contains process chemicals, such as fixing agents, drainage aids, retention aids and flocculants and may contain sizes, dry and wet strength agents, dyes and fillers. The metering of the process chemicals to the paper stock can be carried out by various methods described in the literature. For example, Wochenblatt fur Papierfabrikation 13 (1979), 493-502 discloses the use of cationic polyelectrolytes in combination with bentonite, first bentonite and then the cationic polyelectrolytes being added to the paper stock and it being possible, if required, to subject the paper stock to a shear gradient.
EP-B-0 235 893 discloses that a synthetic cationic polymer having a molar mass of more than 500.000 can initially be added to a paper stock, resulting in the formation of flocks which are then comminuted to microflocks in a subsequent shearing step. Thereafter, bentonite is added and the paper stock containing the process assistants is drained. In the papermaking process disclosed in EP-A-0 335 575, two different water-soluble, cationic polymers are added in succession to the paper stock. Here, a low molecular weight cationic polymer is first metered in as a fixing agent and then a high molecular weight cationic polymer as a flocculant, the paper stock is then subjected to a shear stage with the formation of microflocks, bentonite is added and the paper stock is then drained. However, the formation profile of the papers thus produced is unsatisfactory.